1. Field of the Invention
The present invention relates to flux mapping controlling the positioning of detectors in thimbles of a flux mapping system.
2. Description of the Related Art
Nuclear reactors include incore and excore detectors, i.e., detectors which are positioned inside the nuclear reactor core and detectors which are outside the core but inside the containment. Flux detectors, for example, may be used both inside the core and outside the core. In both cases, the flux detectors measure neutron flux emanating from the fuel in the core. The incore flux detectors are inserted into thimbles which pass through the core. The incore flux detectors are then drawn out of the core at a predetermined rate to produce a flux map along each thimble.
A pressurized water nuclear reactor typically has approximately sixty thimbles, but may have only four detectors. Multiple-path selectors are used to route the detectors into the thimbles. All or a portion, e.g., a quadrant, of the core may be mapped at a given time. Typically, all, e.g., four detectors are inserted into differfent thimbles simultaneously so that the map can be produced as quickly as possible while conditions in the core undergo little change.
Precise positioning of the detectors is important since the purpose of the flux mapping system is to measure neutron flux values at different positions in the core 12. If the positions cannot be determined accurately and precisely, the flux map is not as useful as it should be. In addition, it is important that the flux map be determined as quickly as possible, since conditions in the core can change while the core is being mapped. If one of the four detector drivers 24 is unavailable due to a mechanical breakdown or inaccuracy in its position sensing system, at least one third more time would be required and in many prior art systems twice as much time total would be required to perform flux mapping of the core 12. Also, in the prior art, the ability to detect position of the flux detector could be lost if there was a loss of power when incremental resolvers were used and relatively lower reliability switches were used to verify the position of each detector, preventing use of the detector when the switches malfunctioned.